There are known a dental handpiece having a head section accommodating a bur sleeve in which a dental treatment tool is held, a neck section containing a power transmission shaft, a grip section containing a rotary shaft, and a drive section containing a motor, which sections are connected to each other proximally in this order. Drive of the motor is transmitted via the rotary shaft and the power transmission shaft, and rotates the bur sleeve at high speed.
A conventional handpiece of this type is shown in FIG. 5. A handpiece H has a head 51a for drivingly holding a treatment tool therein, and a neck 51b extending proximally from the head 51a, which two are formed integrally and referred to collectively as a head-neck section 52. To the proximal end of the head-neck section 52, a grip section (not shown) is detachably connected.
The head 51a has a generally cylindrical head housing 54, a gear shaft 55 accommodated in the head housing 54, and a retaining member 56 for rotatably retaining the gear shaft 55 in the head housing 54 as a bearing. The gear shaft 55 is composed of a gear part 57 which has teeth extending upwards and arranged circumferentially in a horizontal plane, and a shaft part 58 extending downwards from the gear part 57 and made of stainless steel. A dust control seal 59 is fit on the shaft part 58 below the retaining member 56, and a cap 62 is fit on the dust control seal 59 to fix the seal 59 and the retaining member 56 in the housing 54.
The dust control seal 59 is a generally annular member having a seal body 60 and an annular lip 61 projecting radially inwardly from the seal body 60 and having a tapered cross section. When external force is applied to the thinned edge of the lip 61, the lip 61 is elastically deformed to sealingly hold the dust control seal 59 against the circumference of the shaft part 58 of the gear shaft 55.
A dental handpiece of this type is disclosed in, for example, JP-11-290342-A.
With the sealing mechanism of such a conventional dust control seal, dust such as tooth debris or abrasive powders tends to enter the contact area between the shaft part 58 of the gear shaft 55 and the dust control seal, and abrades the circumference of the shaft part 58. This may cause entry of dust further into the head housing to damage the gear and lead to rotation instability.